If it's the type I'm familiar with it uses CDS type sensors. CDS sensors
change resistance with the amount of light hitting them (lower
resistance with more light).

It should be dimming when the sensor facing the rear is receiving more
light than the one facing the front, i.e. night with headlights from
behind. When the front facing sensor is receiving enough light, i.e.
daytime it should remain clear. There is also usually a signal from the
transmission for reverse which will override the sensors and insure the
mirror is clear when you are backing up.

If you were to disconnect the rear facing sensor, measure it's
resistance when headlights are hitting it from the rear and then wire a
resistor with that value in place of the sensor it would always "see"
the headlights behind it. It should still remain clear during the
daytime, and when in reverse.

That said, I have the auto dimming rearview on my '97 Chevy truck that
operates in this manner and I've never found a problem with it's
response time.

Not having ever taken one of these apart personally I don't know if
there may be an internal adjustment for response time or sense threshold
that could be adjusted. If you open it up to do the bypass thing check
the board for any adjustments before you start desoldering sensors.

Pete C.